It is generally well known that people suffering from poor visual acuity often find relief by using vision correcting lenses. These may come in the form of corrective contact lenses or corrective lenses fitted into spectacles. In both cases the desired optical characteristics of the lenses (usually sphere power, cylinder and cylinder axis) are determined in the following way: the patient is given a visual acuity test—be it objective (using, for example, an auto-refractometer), or subjective (normally using a trial frame or a phoropter fitted with trial lenses). The outcome of the test is a prescription listing the measured optical properties of the tested eye corresponding to an optimal correction lens for the patient's vision. The prescription can include sets of such optical properties, each normally being referred to as Rx and obtained by at least one measurement of one of the patient's eyes as it gazes at an image (or object), at a standard direction of gaze and distance. In all cases, the optical properties of the tested eye, and indeed the optical properties of the device to be used to correct the malfunctioning eyes are tested when the patient is gazing straight ahead.
Some patients also suffer from Presbyopia (the lack of ability to focus on near objects even if one has corrected vision for objects at infinity), of lacking accommodation. To determine an Rx of a lens for such patient, the patient goes through a further test in which parameters of his eye in the near vision are measured. In this case the patient is usually instructed to read a hand held book using different correctional lenses. The Rx of an optimal correction lens for near vision work is then selected. When obtaining this Rx, the patient is either instructed to look straight ahead when reading or to look in a natural direction of gaze, which is usually to gaze downwards. The Rx found for near work will usually have the same cylinder and axis but have a different value for the sphere power than the Rx for far away objects. The difference between the near and far sphere powers is usually referred to as ‘the addition’. This value constitutes an integral part of the prescription for patients that are to be fitted with progressive or bifocal lenses.